1. Field of the Invention
This invention relates to an image forming apparatus that corrects curl produced on a sheet paper when forming a toner image on the sheet paper.
2. Description of the Related Art
In an electro-photographic image forming apparatus such as a copying machine, a printer and the like, a fixing unit that fixes a toner image formed on a sheet paper by conveying a sheet paper, heating and pressurizing a toner image formed thereon with a pair of fixing rollers is widely used. In this fixing unit, to supply sufficient heat to a toner image, a prescribed nip is formed between the fixing rollers and a satisfactory fixing is obtained by supplying a sheet paper through this nip.
However, after fixing a toner imager, a sheet paper may be curled along the nip shape of the fixing rollers and when tried to discharge a curled sheet paper, it can be bent for defective paper discharge or a sheet paper discharged preceding may disturb the discharge of a succeeding sheet paper. For correcting such curling of a sheet paper, a curl correction unit that controls the pressing force of a pressing roller to press a sheet paper against a belt by a cam is disclosed in Japanese Patent Disclosure No. 6-144679. In addition, an image forming apparatus with two curl correction means arranged at the downstream of a fixing means to correct curl properly according to the direction of curl of recording media is disclosed in Japanese Patent Disclosure No. 2003-66744.
However, both of the above-mentioned curl correction unit and the curl correction means are for correcting sheet paper curl while sheet paper is conveyed linearly and a long path is required.
On the other hand, the downsizing of an image forming apparatus is demanded in recent years in spite of such high function as correspondence to high speed color image forming. Therefore, in order to make the sheet paper conveying distance short, an apparatus to transfer and fix a toner image while conveying sheet paper supplied from a lower paper supply cassette in the vertical direction and then, discharge the sheet paper by guiding in the horizontal direction is developed. In such a small size apparatus, a space for discharge sheet paper conveyed in the vertical direction to the horizontal direction is limited. Therefore, the downsizing is demanded for a curl correction unit for correcting the curl of sheet paper after fixing and its installing space is restricted.
Accordingly, an image forming apparatus that is small in size and can be arranged in a limited space but is capable of properly correcting the curl produced on sheet paper when fixing an image and achieving a satisfactory conveying and discharging sheet paper is demanded.